Generally, fluids such as high-purity chemical products for semiconductors and general-use chemical products and the like are filled into containers such as glass bottles and polyethylene tanks at a production facility, and they are then shipped with a lid attached to the opening for filling and discharging that is formed in this fluid tank. One known method for removing the fluid that has been stored in such a fluid tank is the siphon hose method, in which the fluid is fed to the outside of the container by gas pressure that is produced by introducing a gas, such as air, into the container.
In this method, after removing the lid that has been attached to the opening (below, referred to as a “container inlet”) for filling and discharge from the fluid tank, a plug is installed in the container inlet and then a socket is engaged in the plug. The plug is provided with a siphon hose, which forms a fluid path, and a gas supply duct. A tube for removing fluid and a tube for introducing gas can be connected with the socket. The socket enables respective communication of the tube for removing fluid and the tube for introducing gas with the siphon hose and the gas supply duct of the plug. By engaging the socket in the plug, a primary path for removing fluid and a secondary path for introducing gas are formed in the plug and the socket that are attached to the opening of the container inlet. (Such a fluid tank connector is disclosed, for example, in Japanese Unexamined Patent Application, First Publication No. 2002-59993.)
However, the conventional technology that is disclosed in the above document is a connecting fixture that includes a plug, which is attached to the opening of the container in which a fluid is stored, and a socket, which is connected to this plug. The plug in this case is one that is attached by being threaded into an internal thread that is formed in the opening of the container inlet, and thus, this plug cannot be used in a container in which an external thread is formed on the opening of the container inlet, that is, a container in which the thread for attaching the cap that closes the opening of the container inlet is an external thread.
Note that in this conventional technology, because gas holes are provided in the side surface of a center plug, another problem that has been pointed out is that fluid pooling occurs easily.
In addition, in a plug in which the fluid in a container is removed by using the siphon hose method described above, there are cases in which the fluid that is in the container is circulated by using the siphon hose. In these cases, in addition to the gas path that is used for introducing a gas, the socket, which is used by being coupled to the plug, is provided with a fluid removal path through which the fluid in the container is removed and a fluid return path through which the fluid that has been removed is returned to the container. According to such a plug structure, during fluid circulation, because the fluid removal path and the fluid return path are linked at the outside of the socket, the fluid that has been removed through the siphon hose due to the introduction of gas returns into the container by passing from the fluid removal path through the fluid return path.
During such fluid circulation, when the fluid that is returned into the container from the fluid return path freely falls onto the fluid surface, bubbles are produced due to gas that is incorporated from the fluid surface into the fluid. However, this is not preferable in terms of accurate flow rate measurement and quality control and the like because such bubbles indicate that gas has become trapped in the fluid, such as a chemical product.
In view of the above circumstances, in a plug structure that is used in a container that is filled with a fluid such as a chemical product and the like and is provided with a siphon hose for delivering the fluid to the outside of the container by using the pressure of a gas, the development of a plug structure is desired that can be attached to a container in which the thread of the container inlet for attaching a cap is an external thread, and furthermore, that does not produce bubbles on the fluid surface inside the container even during fluid circulation when the fluid such as a chemical product is being circulated.